This invention relates to measuring instruments. More particularly, the invention relates to an instrument for measuring strain in structural members, incorporating a resonant light modulator, that is particularly suitable for a wide range of environmental conditions, including both cryogenic and high temperatures.
Advances in micromachining technology have given rise to a variety of micro-electromechanical systems (MEMS) including micromachined light modulators for low cost display applications. Such light modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. Representative examples of these light modulators are disclosed in U.S. Pat. Nos. 4,492,435 issued Jan. 8, 1985 to Banton et al. entitled Multiple Array Full Width Electromechanical Modulator, U.S. Pat. No. 4,596,992 issued Jun. 24, 1986 to Hornbeck entitled Linear Spatial Mechanical Light Modulator and Printer, U.S. Pat. No. 5,311,360 issued May 10, 1994 to Bloom et al entitled Method And Apparatus For Modulating a Light Beam; U.S. Pat. No. 5,661,593 issued Aug. 26, 1997 to Engle entitled Linear Electrostatic Modulator, U.S. Pat. No. 5,757,536 issued May 26, 1998 to Ricco et al. entitled Electrically-Programmable Diffraction Grating; U.S. Pat. No. 6,038,057 issued Mar. 14, 2000 to Brazas, Jr. et al. entitled Method and System for Actuating Electro-mechanical Ribbon Elements in Accordance to a Data Stream; and U.S. Pat. No. 6,061,166 issued May 9, 2000 to Furlani et al. entitled Diffractive Light Modulator. Micromachined diffractive light modulators are of particular interest and versatility for strain gauge applications.
Other MEMS devices have been used to sense various physical properties such as acceleration, pressure, mass flow, temperature, humidity, air density or weight. Representative devices are disclosed in U.S. Pat. No. 5,090,254 issued Feb. 25, 1992 to Guckel et al. entitled Polysilicon Resonating Beam Transducers; U.S. Pat. No. 5,275,055 issued Jan. 4, 1994 to Zook et al. entitled Resonant Gauge With Microbeam Driven In Constant Electric Field; U.S. Pat. No. 5,417,115 issued May 23, 1995 to Burns entitled Dielectrically Isolated Resonant Microsensors; and U.S. Pat. No. 5,550,516 issued Aug. 27, 1996 to Burns et al. entitled Integrated Resonant Microbeam Sensor and Transistor Oscillator. The sensors disclosed in these patents are said to operate on the principal that the natural frequency of vibration (i.e. resonate frequency of an oscillating beam or other member) is a function of the strain induced in the member. More particularly, tensile forces tending to elongate the member increase its resonate frequency, while forces tending to compress the member reduce its resonate frequency. The dual vibrating beam transducers disclosed in U.S. Pat. No. 4,901,586 issued Feb. 20, 1990 to Blake et al. entitled Electrostatically Driven Dual Vibrating Beam Force Transducer, are said to operate in an apparently similar manner. All of the above mentioned transducers and sensors use integrated electrical means to sense the motion of the moving member. This limits the design and placement of both the sensors and the associated electronics. There is a need therefore for an improved strain gauge.
The present invention provides a strain gauge for measuring strain in a structural member, including: a light modulator adapted to be attached to the structural member, further including: a plurality of deformable elements, each of said deformable elements having a reflective surface and a resonant frequency that varies as a function of strain on the element; means for exerting a force to the deformable elements to cause them to deform at their resonant frequency between first and second operating states; an optical system for directing incident light onto the light modulator, and directing modulated light from the light modulator to a sensor that provides an output signal that varies as a function of the resonate frequency of said deformable elements; and means for generating a representation of the strain in the structural member from said output signal.
One advantage of the strain gauge of the invention is that the strain of a structural member can be sensed at locations remote from the structural member. Another advantage is the sensitivity of the strain gauge due to the size of its features. Specifically, the strain gauge features are on the order of microns, and it can measure changes in length on the order of nanometers. Other features and advantages of this invention will be apparent from the following detailed description.